Conventionally, there is known a drill in which a point angle of a cutting edge decreases from a tip end toward a rear end of the cutting edge (e.g., refer to Patent Literature 1 and 2). In the drill in Patent Literature 1, the point angle of the cutting edge is an acute angle and decreases in two steps. The point angle on the tip end side is approximately 70° while the point angle on the rear end side is approximately 20°.
According to Patent Literature 1, when a drilled hole penetrating a wall material is to be provided, a large part of the drilled hole is efficiently drilled by the tip end side of the cutting edge having the larger point angle, and at the last step, the hole is cut and broadened by the rear end side of the cutting edge having the smaller point angle without causing breakage, enabling to provide an accurate through hole.
Also, in the drill in Patent Literature 2, the point angle of the cutting edge continuously (smoothly) decreases from the tip end (center position) toward the rear end (maximum diameter position) of the cutting edge, and a clearance angle of the cutting edge continuously decreases from the center position toward the maximum diameter position. According to Patent Literature 2, while abrasion resistance of the cutting edge is improved, the part at the maximum diameter position forms a cutting edge portion for reaming, which enables to conduct reaming by means of the cutting edge.